For GMC units that have no sound for BT or 3rd party apps like YT, or Spotify.
First, check the 3.5 mm jack if it is connected - other end (female) does not need to connect to anything.

Then check Settings>User setting- Background Process- Off>Save & Reboot
Settings>Equalizer>16 Segment EQ>hit Reset
Also Settings>Equalizer>Balance>Reset
If still not working, press RST pinhole using a paper clip
If still not working while on AUX input, try Settings>Install set-666888>look for the option that says Original AUX audio switch and set to Aux Manual>Save and Reboot
Then go to Console and press "Media" switch to the AUX and try the aftermarket sound again

For some Ford Head units, go to Settings>Install Set>666888/8861/1680>F150 Amp - On>Save & Reboot